This invention relates to a robust fault detection method applicable to electronic steering systems for automobiles.
Steering equipment for assisting a driver to steer an automobile is well known in the art. In conventional steering assemblies, the operator controls the direction of the vehicle with the aid of a steering wheel. This wheel is mechanically connected, usually through a gear assembly to the road wheels. To aid the operator, many systems utilize an auxiliary system to generate a force that is transmitted to a steering gear assembly. The additional force reduces the effort required by the operator in changing the direction of the vehicle. Typically, this auxiliary force is generated by either a hydraulic drive or an electric motor.
Control systems are known that provides a vehicle operator with an electric steering assist or electronic steering (or xe2x80x9csteer-by-wirexe2x80x9d) control for a vehicle. In an electric steering assist system, a control input is measured by a control input sensor, e.g., torque sensor. The output of the control input sensor is input into a control unit, which then drives a motor for assisting the driver in turning the steering column and thus turning the front wheels. It has been known to use sensors having a diagnostic output that provides an error signal for purposes of determining whether a sensor failure or fault condition exists. However, prior to the present invention, the control system has not been capable of determining whether a fault condition exists without occasionally generating false-positives. As will be made more clear, the approach taken by the prior art necessarily generated false positives in order to avoid false negatives.
The steer-by-wire control system comprises a steering wheel unit, a control unit, and a steering motor drive that operate together to provide steering control for the vehicle operator. The steering motor drive includes an electric motor for each road wheel steering mechanism and has several sensors including road wheel position sensors and steering wheel sensors. For the same reason mentioned above with respect to an electric steering assist system, the control system has not been capable of determining whether a fault condition exists in these control input sensors without generating false positives.
It would be desirable to provide a system that can accurately distinguish a fault condition from an ordinary transient control output without generating false positives.
The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by providing electronic steer-by-wire and electric steering assist systems with a robust fault detection scheme to determine if a control input sensor in the steering system is faulty. To determine the tolerable error, an error signal is applied at varying levels to determine how long the error can exist without violating pre-established system or vehicle deviation threshold. This testing results in a number of data points of error versus time which can be graphed and which will form a requirements curve.
Once the requirements curve is known, a time-constant and threshold are selected for a low-pass filter and comparator, respectively. The error signal is passed through the low-pass filter and comparator so that the fault discrimination curve approximates the requirements curve without exceeding it.
The above-discussed and other features and advantages will be appreciated and understood by those skilled in the art from the following detailed description and drawings.